A filter bed of granulated activated carbon

The tube can be filled with several kinds of activated carbon, mixed or separately in layers. It is very common to use one carbon only. Activated stone carbon is the most popular.

As regards the filter bed, there are two things that can strongly affect the absorption. The smaller the carbon grains (the granulation) we have in the carbon, the greater the increase in the speed of diffusion (speed of passage / spread through the carbon) so that a more rapid contact takes place both outside and inside the carbon. With granules or pellets of 1-3 mm or larger, there is almost no contact, and impurities do not reach the meso- or micro pores. It does not work. But exactly the same carbon with a finer

granulation works well.

What we want are grains as small as possible. But if the grains are too small, a blockage will occur in the pipe’s carbon bed and no filtration will take place. Soft carbon from peat or wood is usually 0.25-1 mm in size, and harder varieties from coal or coconut shell

around 0.4-0.85 mm. These are very good, suitable grain sizes, giving the alcohol large contact surface with the carbon.

The quality of activated carbon is presently so varied that varieties of carbon with larger grains are usually preferred e.g., 0.4-1.4 mm, to ensure faster filtration. That way, we know it will work – if not perfectly, at least well. The second matter influencing the absorption is the speed of filtration. This is measured in Bed volume per hour (HSV, Hourly Space Velocity), i.e., the quantity of purified alcohol per hour in relation to the volume of the pipe. The volume is easiest measured by filling the pipe with water.

Bed volume per hour (HSV) is usually around 0.25 (very, very slow) when purifying alcohol, while water is usually purified at 2-3 HSV. For a pipe holding 1.7 liters, the maximum purification occurs at 4 dl per hour if the pipe is approx. 40 mm wide and the carbon grain 0.4-1.4 mm in size. If the filtering speed is higher, the carbon sometimes cannot manage proper purification. There are only three ways to speed this up:

1. A wider pipe

2. A longer pipe

3. Smaller carbon grains

It is not possible to have a pipe narrower than 38 mm, because this would create a “wall effect” in the carbon bed, where impurities escape filtration along the wall of the pipe. If we increase the width of the pipe, the alcohol volume per hour is increased without

increasing the flow speed. Within the alcohol industry this filter is over 1 meter wide and pumping the alcohol through the carbon bed from below at 0.25 HSV regulates the HSV.

Method comparison

Using the method I describe and a Prestige activated carbon or another carbon of the same capacity, one can often filtrate much faster then 0.25 HSV.